This invention relates to multiple charged particle beams, for use in inspection or lithography.
It is well known to inspect semiconductor devices, for instance, those fabricated on semiconductor wafers, using a scanning electron beam inspection tool. Advantageously such electron beam inspection is not constrained by the optical (light) diffraction limits encountered by optical inspection techniques. Therefore electron beam inspection is considered useful for next generation (critical dimension less than 0.25 microns) inspection for semiconductor manufacturing. Currently commercially available electron beam-based inspection machines use a single electron beam column, based on the principle of scanning electron microscopy. The single electron beam is raster scanned over the areas of interest on a fabricated semiconductor wafer. Secondary and back-scattered electrons generated by the incident beam are detected and registered pixel-by-pixel using conventional image processing techniques to reconstruct an image of the inspected region.
Low throughput is a significant obstacle in such machines, because the images are acquired as described above pixel-by-pixel in a sequential manner. Low throughput significantly raises inspection costs and hence is undesirable.
In accordance with this invention, an apparatus and associated method are provided for electron (or other charged particle) multiple beams for inspection or lithography of microstructures such as features formed on a semiconductor wafer. Applications include high-speed imaging, charged particle lithography and defect inspection of micro-fabricated structures including defect inspection of integrated circuit die on semiconductor wafers, masks or reticles for microfabrication, flat panel displays, and micro-electro-mechanical (MEMs) devices during and after manufacture. In particular, one embodiment is directed to high-speed defect inspection with a cost-effective and scalable multi-beam approach. The inspected features include, for instance, gates, contacts, vias, interconnects, and other semiconductor or micro-machined structures, including diffused regions.
The apparatus (tool) includes array of charged particle beam columns, each directing its beam onto the workpiece (e.g., semiconductor wafer). Each beam column includes its own electron source (emitter) or ion source; a gun lens downstream of the source to focus the charged particles into the beam; a suitable beam aperture and an associated isolation valve to preserve the vacuum; column alignment and astigmatism correction compensation elements; a secondary (back-scattered) electron or particle detector and an objective lens. (It is to be understood that generally here the terms optics, lens, etc., apply to charged particle optics, rather than to light optics.)
The array of beam columns, a 3xc3x973 or 5xc3x975 beam array for instance, also includes a single objective lens coil, which is an electro-magnetic coil surrounding the individual objective lens pole pieces of all the beam columns in the array. This arrangement provides superior (lower cost and greater stability) optical properties and high electron beam density.
One limitation of the throughput of a prior art single column system for inspection is the individual detector/data path performance, which typically, in terms of data flow, is less than 100 Mhz per second. The present multibeam (multicolumn) system, in contrast, does not have this limitation because the required bandwidth for each detector is reduced by the number of columns. The distribution of the column array across a region under inspection (or being imaged for lithography) of the workpiece makes it possible to implement a stage with travel a fraction of the size of the workpiece. This reduces the footprint of the associated vacuum chamber to xc2xd to ⅓ of the size required in the prior art, e.g., a typical chamber holding a 300 mm wafer stage is 1.5 to 2 meters long but a typical 200 mm wafer stage chamber is about 50% smaller. The smaller 200 mm stage can be used with the column array to inspect or fabricate a 300 mm wafer.
Also included, in one embodiment, are xe2x80x9cnullxe2x80x9d (dummy) beam columns surrounding the column array. That is, the perimeter portions of the array contain beam columns that may omit certain elements (for instance, electron emitter and detectors) but do contain other elements (for instance, the magnetic objective lens pole pieces, the magnetic gun lens pole pieces, and magnetic deflection elements) as do the active columns. The null columns are included to improve the optical properties of the active columns.
An associated method uses multiple beam columns each with its own detector for micro-structure inspection. This can be done using a step-and-scan scheme for inspection or continuously moving the stage supporting the workpiece under inspection. If one of the beam columns fails, the system control system registers this failure. The area intended to be inspected by the failed beam column can be inspected using one of the other (active) columns or be inspected using a conventional single column system that is designed to inspect a small portion of a wafer.